1. Technical Field
This invention relates to a method and apparatus for detecting radiated thermal energy and, more particularly, to interferometric techniques for accomplishing this function.
2. Background
Devices that sense infrared (IR) radiation having wavelengths beyond about 5 microns are generally classified as thermal or photon detectors. Photon detectors such as those made out of HgCdTe material are typically very fast and highly sensitive. Unfortunately, known photon detectors that are sensitive to wavelengths over about 5 microns usually require additional cooling apparatus to maintain them at a specific operating temperature, typically 77 degrees Kelvin. In addition, many of these known devices can be destroyed if subjected to high temperatures, such as above around 333 degrees Kelvin.
In constrast, thermal detectors such as bolometers, pyoelectric devices, Golay cells and thermo-couples are relatively slow in comparison with photon detectors. On the other hand, known thermal detectors can operate over a wider temperature range and do not need external cooling apparatus to maintain them at given temperatures. Thus, thermal detectors can generally be manufactured in relatively small, lightweight and rugged packages.
Interferometric temperature sensors using optical fibers have been described in the literature. See, for example, Corke et al., "All-Fibre Michelson Thermometer", Electronics Letters, Vol. 19, No. 13 (June 1983); Sigel Jr., "Minitutorial on Fiber Optic Sensors", presented at the Eighth Conference on Optical Fiber Communication (Feb. 13, 1985); DePaula et al., "Fiber Optic Sensor Overview", Fiber Optic and Laser Sensors III, SPIE Vol. 566 (1985); and Jackson, "Tutorial on Fiber Optic Sensors", presented at the Third International Conference on Optical Fiber Sensors (Feb. 13, 1985).
There exists a need for a detector that can accurately respond quickly to radiated IR energy in the 6-30 micron region that does not respond to conducted or convected thermal energy. One potential application is in a fire sensing and suppression system designed to respond to fires or explosions that radiate IR energy in this spectral band while not being falsely activated by conducted or convected thermal energy due to false alarm sources such as hot gas vents. The interferometric temperature sensors described above do not fulfill this need because while they are effective in sensing thermal energy in their immediate vicinity, they are relatively insensitive to radiated energy from remote sources of heat. Thus, they will not respond effectively to fires or explosions that are some distance away.